Date: Fri, 25 Feb 2022 10:23:04 -0600
From: Justin Pryzby <pryzby@telsasoft.com>
To: Magnus Hagander <magnus@hagander.net>
Cc: pgsql-hackers@lists.postgresql.org
Subject: Re: Add parameter jit_warn_above_fraction
Message-ID: <20220225162304.GD9008@telsasoft.com>
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On Fri, Feb 25, 2022 at 04:16:01PM +0100, Magnus Hagander wrote:
> +	{
> +		{"jit_warn_above_fraction", PGC_SUSET, LOGGING_WHEN,
> +			gettext_noop("Sets the fraction of query time spent on JIT before writing"
> +						 "a warning to the log."),
> +			gettext_noop("Write a message tot he server log if more than this"
> +						 "fraction of the query runtime is spent on JIT."
> +						 "Zero turns off the warning.")
> +		},
> +		&jit_warn_above_fraction,
> +		0.0, 0.0, 1.0,
> +		NULL, NULL, NULL
> +	},

Should be PGC_USERSET ?

+                       gettext_noop("Write a message tot he server log if more than this"                                                                                                                                         

to the

+       if (jit_enabled && jit_warn_above_fraction > 0)                                                                                                                                                                            
+       {                                                                                                                                                                                                                          
+               int64 jit_time =                                                                                                                                                                                                   
+                       INSTR_TIME_GET_MILLISEC(portal->queryDesc->estate->es_jit->instr.generation_counter) +                                                                                                                     
+                       INSTR_TIME_GET_MILLISEC(portal->queryDesc->estate->es_jit->instr.inlining_counter) +                                                                                                                       
+                       INSTR_TIME_GET_MILLISEC(portal->queryDesc->estate->es_jit->instr.optimization_counter) +                                                                                                                   
+                       INSTR_TIME_GET_MILLISEC(portal->queryDesc->estate->es_jit->instr.emission_counter);                                                                                                                        
+                                                                                                                                                                                                                                  
+               if (jit_time > msecs * jit_warn_above_fraction)                                                                                                                                                                    
+               {                                                                                                                                                                                                                  
+                       ereport(WARNING,                                                                                                                                                                                           
+                                       (errmsg("JIT time was %ld ms of %d ms",                                                                                                                                                    
+                                                       jit_time, msecs)));                                                                                                                                                        
+               }                                                                                                                                                                                                                  
+       }                                                                                                                                                                                                                          


I think it should be a NOTICE (or less?)

Is it more useful if this is applied combined with log_min_duration_statement ?

It's easy to imagine a query for which the planner computes a high cost, but
actually runs quickly.  You might get a bunch of WARNINGs that the query took
10 MS and JIT was 75% of that, even if you don't care about queries that take
less than 10 SEC.

I should say that this is already available by processing the output of
autoexplain.

-- 
Justin